Showing papers in "Monthly Notices of the Royal Astronomical Society in 1991"
TL;DR: In this article, the observed spectra consist of both direct radiation from the primary X-ray source and reflection from the surrounding accretion flow, and features imprinted in it by photoabsorption, iron fluorescence and Compton scattering.
Abstract: Weak, broad emission lines due to low-ionisation stages of iron, and other spectral features, have recently been observed in the X-ray spectra of Active Galactic Nuclei and in some Galatic X-ray binaries. These features are due to X-ray irradiation of relatively cold, dense gas very close to the central compact object. The observed spectra consist of both direct radiation from the primary X-ray source and reflection' from the surrounding accretion flow. The reflected spectrum has features imprinted in it by photoabsorption, iron fluorescence and Compton scattering. The strength, shape and broadening of this reflected spectrum is a diagnostic of the geometry, ionisation state and iron abundance of the accretion flow, and through timing studies, may yield the mass of the central object.
1,150 citations
TL;DR: In this article, a sample of 10 galaxies is considered in view of two suggested explanations for the discrepancy between the luminous mass and the conventional dynamical mass of galaxies: dark haloes and modified Newtonian dynamics (MOND).
Abstract: Strict criteria are applied to the sample of spiral galaxies with measured rotation curves in order to select those objects for which the observed rotation curve is an accurate tracer of the radial force law. The resulting sub-sample of 10 galaxies is then considered in view of two suggested explanations for the discrepancy between the luminous mass and the conventional dynamical mass of galaxies: dark haloes and the modified Newtonian dynamics (MOND). This is done by means of least-squares fits to the rotation curves. Three-parameter dark-halo models (M/L for the visible disc, the core radius and the asymptotic circular velocity of the halo) work well in reproducing the observed rotation curves, and it is found that, for the higher luminosity galaxies, the visible matter dominates the mass distribution within the optically bright disc. However, in the low-luminosity gas-rich dwarfs the dark component is everywhere dominant. MOND, with one free parameter, (M/L for the visible disc) generally works well in predicting the form of the rotation curves, in some cases better than multi-parameter dark-halo fits. If the distance to the galaxy is also taken as a free parameter, then the MOND fits are as good as three parameter dark-halo models and, with one exception, the implied distances are consistent with the adopted distances within the probable uncertainty in the distance estimates. Restricting the number of parameter in dark-halo models by making use of the disc-halo coupling does not produce satisfactory fits to the rotation curves. The overall conclusion is that MOND is currently the best phenomenological description of the systematics of the discrepancy in galaxies.
967 citations
TL;DR: In this article, the authors explored the possibility of measuring the density parameter OMEGA-0 and the cosmological constant lambda-0 = LAMBDA/(3H0(2)) using dynamical tests in linear and non-linear theory.
Abstract: The possibility of measuring the density parameter OMEGA-0 and the cosmological constant lambda-0 = LAMBDA/(3H0(2)) using dynamical tests is explored in linear and non-linear theory. In linear theory we find that the rate of growth of the perturbations at the present epoch is approximated by f(z = 0) almost-equal-to-OMEGA-0(0.6) + 1/70 lambda-0(1 + 1/2 OMEGA-0). Therefore, dynamical tests such as infall around clusters and dipoles at the present epoch do not distinguish well between universes with and without a cosmological constant. At higher redshifts, the perturbations also depend mainly on the matter density at a particular epoch, f(z) almost-equal-to OMEGA-0.6(z), which has a strong dependence on lambda-0 at z almost-equal-to 0.5-2.0. Therefore, information on both parameters can be obtained by looking at clustering at different redshifts. In practice, however, the other observables also depend on the cosmology, and in some cases conspire to give a weak dependence on lambda-0. By using the non-linear spherical infall model for a family of Cold Dark Matter (CDM) power-spectra we also find that dynamics at z = 0 does not tell much about lambda-0. At higher redshifts there is unfortunately another conspiracy between conventional observables, which hides information about lambda-0. The final radius of a virialized cluster (relative to the turnaround radius) is approximated by R(f)/R(ta) almost-equal-to (1 - eta/2)/(2 - eta/2), where eta is the ratio of LAMBDA to the density at turn-around. Therefore a repulsive-LAMBDA gives a smaller final radius than a vanishing-LAMBDA.
720 citations
TL;DR: In this paper, the use of a lognormal (LN) random field is discussed as a model for the distribution of matter in the universe, and a number of difficulties with the statistical treatment of density perturbations are revealed by an analysis of this model.
Abstract: The use of a lognormal (LN) random field is discussed as a model for the distribution of matter in the Universe. A number of difficulties with the statistical treatment of density perturbations are revealed by an analysis of this model. In particular, the LN model is not completely specified by its moments. The reason for which this could be true for the actual matter field is explained. It is also shown that the usual method of representing the three- and four-point correlation functions of galaxies, in terms of the parameters Q and R, is not useful for discriminating between Gaussian and non-Gaussian fluctuations, and better parameterizations are proposed in terms of the skewness and kurtosis of the three- and four-point distributions, respectively.
594 citations
TL;DR: In this article, a coherent picture of the Galaxy's H I, CO and CS emissions in the region |l| ≤ 10°, |b| ≤ 0.5° is constructed.
Abstract: A coherent picture is constructed on the Galaxy's H I, CO and CS emissions in the region |l| ≤ 10°, |b| ≤ 0.5°. The flow of gas at the Galactic Centre is dominated by a bar that has corotation at r = 2.4 ± 0.5 kpc, which we view at an angle of 16 ± 2° from its major axis. The first CO emission arises where gas is obliged to switch from x 1 orbits to x 2 orbits in the notation of Contopoulos. This gives rise to a shock and a clear signature in the (l, v) diagram. The great Galactic Centre molecular clouds such as Sgr B are on x 2 orbits.
583 citations
TL;DR: In this article, the relative occurrence of narrow-lined and broad-lined AGN in optical, IR, X-ray, and radio-selected samples is discussed. But the authors do not consider the effect of the source power on the torus geometrical thickness of AGN.
Abstract: I discuss evidence concerning the relative occurrence of narrow-lined and broad-lined active galactic nuclei (AGN), in optical, IR, X-ray, and radio-selected samples. Both narrow-lined AGN and reddened broad-lined AGN occur more frequently at lower source powers. There is marginal evidence that narrow-lined AGN were more common in the past. Narrow-lined objects have weaker [O III] at a given radio power. These data are inconsistent with the simplest unified scheme where a similar thick molecular torus surrounds all AGN, and with the simplest modification, that the torus geometrical thickness is a function of source power.
472 citations
TL;DR: In this article, the Press-Schechter theory is extended in order to focus on the subset regions that will eventually collapse to form cluster of a set mass, and an application to the evolution of groups of galaxies is realised.
Abstract: The Press-Schechter theory provides a simple analytical description for the evolution of gravitational structure in a hierarchical univers. The original theory is extended in order to focus on the subset regions that will eventually collapse to form cluster of a set mass. The conditional multiplicity function of groups at an epoch with redshift z, given that they are bound into an object of a particular mass at the present epoch, is determined and combined with the present distribution of group masses in order to obtain the joint multiplicity function. An application to the evolution of groups of galaxies is realised.
429 citations
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TL;DR: In this article, the authors show that the flow caused by a supersonic gas jet is self-similar under certain conditions and that the type of similarity solution depends on the density distribution of the gas through which the jet propagates.
Abstract: General arguments and numerical calculations are used to show that the flow caused by a supersonic gas jet is self-similar under certain conditions. If we assume that the jet has a high initial Mach number and is generated in a region small compared to its length, then the type of similarity solution depends on the density distribution of the gas through which the jet propagates. If this density decreases faster than 1/R 2 , where R is the distance from the source, then the length of the jet increases linearly with time and it may evolve into a classical double if it subsequently encounters a region of higher density.
397 citations
TL;DR: In this paper, the spectral energy distributions of 60 galaxies for which 8-13 μm and in some cases 17-23 μm, spectra have been obtained, and these IR bright galaxies are discussed in terms of their spectral properties in the mid-IR and the relationship with their far-IR properties.
Abstract: The spectral energy distributions are presented for 60 galaxies for which 8-13 μm, and in some cases 17-23 μm, spectra have been obtained. These IR bright galaxies are discussed in terms of their spectral properties in the mid-IR and the relationship with their far-IR properties. Almost all of the galaxies can be placed in one of three classes on the basis of their 8-13 μm spectra : those dominated by the family of narrow bands between 3-13 μm, those with featureless continua and those that display the silicate absorption band. H II region galaxy nuclei, i.e., those which contain powerful nuclear H II regions and whose mid-IR spectra are dominated by emission in the family of narrow emission bands display little dispersion in their IR properties.
308 citations
TL;DR: In this article, the properties of geometrically thin viscous Keplerian excretion discs around Be stars are analyzed and the inner boundary of the disc is assumed to be located at the equatorial surface of the central star.
Abstract: The properties of geometrically thin viscous Keplerian excretion discs around Be stars are analyzed. The inner boundary of the disc is assumed to be located at the equatorial surface of the central star. It is assumed that angular momentum is transferred by some mechanism from the interior to the equatorial surface of the central star and hence matter is supplied at the inner boundary of the disc at a certain rate (excretion rate). The matter drifts outward in the disc because the angular momentum of the matter is exchanged by the effect of viscous stress. For mass excretion rates similar to the mass-loss rate estimated for Be stars, the irradiative heating by the central star is as important for the disc structure as the viscous heating in the disc
280 citations
TL;DR: In this paper, a prescription for quantifying the galaxy distortion and a propagation equation for its evolution in an inhomogeneous universe is derived, and a measurement of galaxy distortion over patches of the sky up to a few square degrees in size is given.
Abstract: Inhomogeneity in the distribution of mass in the Universe on scales ≤ 100 Mpc can generate a coherent shear distortion or polarization of the images of background galaxies. This distortion may be measurable over patches of the sky up to a few square degrees in size. If this distortion is measured, or conversely, if its magnitude is limited, it should help us understand the degree to which luminosity traces the underlying mass over cosmological scales. A prescription is given for quantifying the galaxy distortion and a propagation equation for its evolution in an inhomogeneous universe is derived
TL;DR: In this paper, X-ray observations of 45 clusters of galaxies made with the European Xray Observatory Satellite, EXOSAT, are presented, and the 1−20 keV spectra obtained constitute the largest homogeneous spectral sample presently available.
Abstract: X-ray observations of 45 clusters of galaxies made with the European X-ray Observatory Satellite, EXOSAT, are presented The 01−20 keV spectra obtained constitute the largest homogeneous spectral sample presently available Here the analysis of the spectral and imaging data is described A significant scatter in the correlation between the X-ray luminosity and temperature, which can be interpreted as variations in the ratio of the gas mass to total mass amongst the clusters, is found The measured iron abundances are consistent with a canonical average value of 03 solar
TL;DR: In this paper, the properties of external accretion discs (discs with a central source of angular momentum) are explored both analytically and numerically, and an illustrative example of the effect of a disc of material around a binary star on the stellar separation is considered.
Abstract: The properties of external accretion discs (discs with a central source of angular momentum) are explored both analytically and numerically. An illustrative example of the effect of a disc of material around a binary star on the stellar separation is considered. A Greens-function-type solution is considered, in which an initial ring of matter is put in orbit around the central binary.
TL;DR: In this article, the structure of accretion discs within binary systems is influenced by the excitation of resonances within the disc, which can be used to explain the superhump phenomenon of SU UMa dwarf novae in superoutburst.
Abstract: The structure of accretion discs within binary systems is shown to be influenced by the excitation of resonances within the disc. Of particular importance is that near the 3:1 commensurability with the stars' orbit. This can be used to explain the superhump phenomenon of SU UMa dwarf novae in superoutburst. This resonance can only appear for mass ratios which satisfy M 1 /M 2 ≤ 0.25-0.33, for larger mass ratios the available resonances are weaker and of the wrong form to produce the superhump phenomenon. The mass-transfer stream is shown to be an important contributor to growth rate of the resonance.
TL;DR: In this paper, a dynamical model for 37 bright elliptical galaxies for which high-quality photometrical and both major and minor axis kinematical data are available in the literature is constructed.
Abstract: Axisymmetric dynamical models are constructed for 37 bright elliptical galaxies for which high-quality photometrical, and both major and minor axis kinematical data are available in the literature. The models are of the type used previously by Binney, Davies & Illingworth and van der Marel, Binney & Davies. The projected kinematics are predicted from the observed surface photometry, assuming a constant mass-to-light ratio, and a velocity ellipsoid with σ r =σ θ [i.e. f=f(E,L z )].
TL;DR: The results obtained for clusters of galaxies using X-ray data from the European X-Ray Observatory Satellite, EXOSAT, are compared with the optical, infrared and radio properties of the clusters taken from the literature.
Abstract: The results obtained for clusters of galaxies using X-ray data from the European X-ray Observatory Satellite, EXOSAT, are compared with the optical, infrared and radio properties of the clusters taken from the literature. A number of strong correlations are found. The ratios of the intracluster medium (ICM) temperature and the cluster velocity dispersion, β, have a mean of less than one, in agreement with values obtained from the surface brightness distribution of the X-ray emission, thus resolving the Beta problem
TL;DR: In this article, the authors examined the possibility that stars in the nuclei of galaxies could be ground down into short-period orbits, by interaction with a disc around a central massive black hole.
Abstract: We examine the possibility that stars in the nuclei of galaxies could be ground down into short-period orbits, by interaction with a disc around a central massive black hole. A star on a highly eccentric orbit, which passed close to the hole, would lose energy and momentum in passing through the disc. The cumulative effect of many passages would bring the star into a circular orbit corotating with the disc. The star would then either open up a gap in the disc (if the disc is thinner than the star's Roche radius and the viscosity in the disc is sufficiently low), or accrete material from the disc.
TL;DR: In this paper, the effects of finite core radii potential ellipticity on lensing cross sections, the predicted distribution of lens image separations compared to observational angular resolutions, and the preferential inclusion of lens systems in flux limited samples are considered in order to facilitate more realistic predictions of multiple image galaxy-quasar lensing frequencies.
Abstract: Four issues - (1) the best currently available data on the galaxy velocity-dispersion distribution, (2) the effects of finite core radii potential ellipticity on lensing cross sections, (3) the predicted distribution of lens image separations compared to observational angular resolutions, and (4) the preferential inclusion of lens systems in flux limited samples - are considered in order to facilitate more realistic predictions of multiple image galaxy-quasar lensing frequencies. It is found that (1) the SIS lensing parameter F equals 0.047 +/-0.019 with almost 90 percent contributed by E and S0 galaxies, (2) observed E and S0 core radii are remarkably small, yielding a factor of less than about 2 reduction in total lensing cross sections, (3) 50 percent of galaxy-quasar lenses have image separations greater than about 1.3 arcsec, and (4) amplification bias factors are large and must be carefully taken into account. It is concluded that flat universe models excessively dominated by the cosmological constant are not favored by the small observed galaxy-quasar lensing rate.
TL;DR: In this article, the observed polarization properties of an extended polarized radio source situated behind an irregular foreground Faraday screen are studied and the polarization behavior is not strongly dependent on the assumed power spectrum of rotation measure fluctuations and generally behaves as a power law of wavelength.
Abstract: The observed polarization properties of an extended polarized radio source situated behind an irregular foreground Faraday screen are studied. Depolarization laws are derived for both a uniform and a linear source. The polarization behaviour is not strongly dependent on the assumed power spectrum of rotation measure fluctuations and generally behaves as a power law of wavelength when the root mean square rotated angle exceeds unity, a much slower fall off than given by Burn's formula
TL;DR: In this article, the form of the B and R galaxy number-magnitude count relations in 12 independent fields for 21 m < B < 25 m and 19 m < R < 23.5 m is determined, using the Prime Focus CCD Camera at the Isaac Newton Telescope.
Abstract: The form of the B and R galaxy number-magnitude count relations in 12 independent fields for 21 m < B < 25 m and 19 m < R < 23.5 m is determined, using the Prime Focus CCD Camera at the Isaac Newton Telescope. The average galaxy count relations lie in the middle of the wide range previously encompassed by photographic data. In the independent data set, reasonable agreement are found with the CCD galaxy counts of Tyson J. A. (1988, Astron. J., 96).
TL;DR: In this paper, a model explaining the existence and properties of the circular polarization in pulsar emission is offered, based on the properties of waves excited by possible plasma instabilities, and the cyclotron instability developing due to the anomalous Doppler effect is carefully investigated.
Abstract: A model explaining the existence and properties of the circular polarization in pulsar emission is offered. It is based on the properties of waves excited in pulsar magnetospheres by possible plasma instabilities. The cyclotron instability developing due to the anomalous Doppler effect is carefully investigated. The following characteristics of the emission polarization are explained : if circular polarization exists it should be confined to small angles and observed in the core-type emission; the maximum intensity of circular polarization in the profile window is near the intensity maximum of the core-type emission; the sense-reversals of circular polarization in some pulsars and their absence in others; the correlation between circular polarization and position-angle swings; the behaviour of circular polarization in PSR 1541+09, 1702−19 and others
TL;DR: In this article, it was demonstrated that the LSQ covariance matrix does not account for correlation of residuals from the fit. And the Rayleigh resolution criterion is insensitive to signal-to-noise ratio and thus does not reflect quality of observations.
Abstract: Periods of oscillation are frequently found using one of two methods: least-squares (LSQ) fit or power spectrum. Their errors are estimated using the LSQ correlation matrix or the Rayleigh resolution criterion δv R = 1/ΔT, respectively. It is demonstrated that both estimates are statistically incorrect. On the one hand the LSQ covariance matrix does not account for correlation of residuals from the fit. Neglect of the correlations may cause large underestimation of the variance. On the other hand the Rayleigh resolution criterion is insensitive to signal-to-noise ratio and thus does not reflect quality of observations. The correct variance estimates are derived for the two methods.
TL;DR: The effect of unresolved binary stars on the stellar luminosity function that results from a given mass function is quantified in this paper, where it is assumed that both component masses of a binary system are chosen independently from the same mass function.
Abstract: The effect of unresolved binary stars on the stellar luminosity function that results from a given mass function are quantified. It is such that previous studies have significantly underestimated the numbers of low-mass stars. If it is assumed that both component masses of a binary system are chosen independently from the same mass function, unresolved binary systems can account for the difference between the shapes of those luminosity functions, deduced from photographic samples and that deduced from nearby stars with trigonometric parallaxes. The best results are obtained if all stars are binary systems with two hydrogen-burning components
TL;DR: In this paper, an all-sky sample of radio galaxies at redshifts z ≤ 0.1 is used to study clustering in the Universe on scales up to several hundred Mpc.
Abstract: An all-sky sample of radio galaxies at redshifts z ≤ 0.1 is used to study clustering in the Universe on scales up to several hundred Mpc. The two-point correlation function for these galaxies is consistent with their high optical luminosity and location in moderately richy environments: ξ(r) ⇒ (r/11 h −1 Mpc) −1.8 . Direct methods for obtaining the power spectrum of the density field traced by the radio galaxies are discussed, taking into account the selection function of the sample. The results of the power-spectrum analysis indicate that the distribution of radio galaxies is more uniform on very large scales than would be predicted from an extrapolation of the power-law clustering found on small scales
TL;DR: In this article, a capturecollision-cascade model was developed for calculating recombination spectra of He I in nebular conditions and the results of computations at temperatures from 312.5 to 20000 K and electron densities of 10 2, 10 4 and 10 6 cm −3 were presented.
Abstract: A capture-collision-cascade model has been developed for calculating recombination spectra of He I in nebular conditions. The method of solution closely follows the treatment of Brocklehurst M. (1972, Mon. Not. Roy. ast. Soc., 224). The results of computations at temperatures from 312.5 to 20000 K and electron densities of 10 2 , 10 4 and 10 6 cm −3 are presented. The singlets have been treated in both case A and B conditions, the triplets in case B only. The He I results use newer data than the calculations of Brocklehurst and highlight an error in his work. These calculations can be used to interpret the line emissions from He I in the optical and infrared bands
TL;DR: In this paper, the authors describe the convective driving mechanism which excites the oscillations in the outer layers of the variable white dwarfs in a state of partial ionization.
Abstract: The outer layers of the variable white dwarfs are in a state of partial ionization. During the pulsation cycle the base of the ionization zone is strongly heated by the radiative layers below, in phase with the pressure perturbation. If this excess heat is not quickly lost at the surface, then the driving effect is strong. The surface flux perturbation tends to be small and delayed in phase because the surface flux is remarkably insensitive to temperature changes in the deeper layers of the ionization zone. This insensitivity is closely associated with the well known inward divergence of the solutions for the equilibrium thermal structure in the convective layers. The mechanism which excites the oscillations could be called convective driving
TL;DR: In this paper, a sample of 141 IRAS objects, selected on the basis of their IRAS colours and lying in two strips across the Galactic Bulge between 7°< |b| < 8° and 345°< l < 15°, was presented for new-infrared JHKL photometry.
Abstract: New-infrared JHKL photometry is presented for a sample of 141 IRAS objects, selected on the basis of their IRAS colours and lying in two strips across the Galactic Bulge between 7°< |b| < 8° and 345°< l < 15°. Natures of the sources, and periods and bolometric magnitudes of the variables are determined. Properties of the 113 Mira stars identified in the sample are analysed.
TL;DR: In this article, the authors discuss various interpretations of the effect found in powerful double radio sources with one-sided jets whereby the depolarization is systematically stronger on the counter-jet side.
Abstract: We discuss various interpretations of the effect found in powerful double radio sources with one-sided jets whereby the depolarization is systematically stronger on the counter-jet side. Depolarization by gas located within the source is unlikely to be the cause of the asymmetry. Line-emitting gas, though often associated with depolarization in radio galaxies, is not systematically stronger on the counter-jet side. We interpret the asymmetry in terms of a halo of hot gas surrounding the radio source. If the visible jet is on the near side of the source, as required for the relativistic beaming hypothesis, it will be seen through less depolarizing gas
TL;DR: A generalization of the Osipkov-Merritt inversion of the fundamental integral equation for anisotropic spherical galaxies is obtained in this article, which yields a two-parameter distribution function with arbitrary degrees of both radial and tangential velocity anisotropies.
Abstract: A generalization of the Osipkov-Merritt inversion of the fundamental integral equation for anisotropic spherical galaxies is obtained. For any density profile this inversion yields a two-parameter distribution function with arbitrary degrees of both radial and tangential velocity anisotropies. Therefore, for the first time, exact analytical models can be found which generate given density profiles and possess tangential velocity anisotropies. This property disposes of the well-known radial orbit instability encountered by many models of anisotropic spherical galaxies
TL;DR: In this paper, a simple explanation for this similarity can be found if the mass loss, by stellar wind, from the Roche filling star is the driving mechanism behind mass transfer in close binary systems.
Abstract: Stars in close binary systems can suffer two kinds of mass change : 1) mass transfer between the stars, and 2) mass loss completely from the system. The former process occurs when one the stars fills its Roche lobe while the latter may be in the form of a stellar wind. When both of these exist together, observational estimates indicate that the mass-transfer rate and the mass-loss rate are of the same order. A simple explanation for this similarity can be found if the mass loss, by stellar wind, from the Roche filling star is the driving mechanism behind mass transfer